Justification of the Transition to Erbium Fuel in VVER-1000 Type Reactors Based on a Comparative Analysis of Reactivity Coefficients for Gadolinium and Erbium Burnable Absorbers

The paper presents the neutronic justification of replacement of the burnup gadolinium absorber placed in a small number of fuel elements by erbium placed in all fuel elements of the fuel assembly. A scheme of placement of fuel elements is chosen for the gadolinium absorber that models their real arrangement in fuel assemblies with maximum concentration of gadolinium in fuel elements (8%). In the selected scheme, the center fuel element is symmetrically surrounded by two rows of fuel elements with the total number of fuel elements of 18. When selecting the weight content of erbium in fuel elements, not only is the total compensated reactivity reserve for the campaign taken into account but also intrinsic safety properties such as the density and total temperature coefficients of reactivity of the fuel loading with respect to the coolant, which allows reducing the negative influence of erbium on the burnup of unloaded fuel. In addition, the homogeneous arrangement of erbium in fuel elements leads to a uniform distribution of energy release in comparison with the gadolinium absorber. The comparative analysis of neutronic characteristics of the reactor is carried out for the considered absorbers on unit cells and polycells taking into account simplified burnup models at partial reloads without rearrangements of fuel assemblies. Repetitive structures consisting of three fuel assemblies with different irradiation durations are formed under the condition of application of threefold partial fuel reloading in the core. The results obtained clearly demonstrate that the reactivity coefficients for the erbium absorber have higher values than for the gadolinium variant under the condition of equal compensation of excess reactivity. Taking this factor into account, it is possible to select such a weight content of erbium in fuel elements when losses in burnup will be insignificant.